Gaskets for fuel caps have recently come to be formed from fluororubbers having high gasoline permeation resistance to comply with the requirement for fuel permeation control (Japanese Patent Application Publication No. JP-A-2004-60819). The gasket for a fuel cap has a fluororubber base surface coated with a sliding-treated layer to improve the sliding property of fluororubber so that the gasket may be able to slide smoothly on the seal rest of a filler neck when the fuel cap is tightened about the filler neck.
The inventors of the present invention have previously proposed in JP-A-2004-60819, as a sliding-treated layer, a baked coating of a sliding treatment agent comprising an aqueous emulsion containing a solid lubricant (e.g. a fluororesin powder), an urethane resin as a matrix (base material) and a reactive group-coupled alkyl trialkoxysilane compound (ATAS) (a silane coupling agent) as an adhesion improver. The combination of a urethane resin and ATAS gives a sliding-treated layer having an improved adhesion to fluororubber. Its improved adhesion is presumably due to the strength of the urethane resin improved by its thermosetting reaction during the baking of the layer and the hydrogen bonding between the ATAS and the fluororubber composition.
Referring to FIG. 8, a gasket 51 employed for a fuel cap 1 in an automobile is in most of the cases a ring having a substantially C-shaped cross section and has a short lip 52 which is deformable only to a small extent, for example, 2 mm when pressed against a filler neck 2. The gasket 51 as described does not present any problem in particular even if its fluororubber base surface may be coated with a sliding-treated layer containing a urethane resin as the matrix.
Referring to FIG. 9, however, there has recently been developed a gasket 51 having a substantially J-shaped cross section and having a long lip 52 which is deformable to about twice an extent, for example, 4.5 mm so that a fuel cap 1 may be put on or off by turning more easily and may be closed more tightly. The gasket 51 as shown in FIG. 9 is likely to present problems, such as interfacial separation and cracking, if its fluororubber base surface is coated with a sliding-treated layer containing a urethane resin as the matrix. FIG. 10 is an enlarged cross sectional view of a surface portion thereof.
FIG. 10 shows a fluororubber base 53 and a sliding-treated layer 54 composed of a silane coupling agent 55, a urethane resin as a matrix 56 and a fluororesin (polytetrafluoroethylene, or PTFE) powder as a solid lubricant 57. When the gasket 51 simply contacting the filler neck 2 as shown in FIG. 9A is pressed against it as shown in FIG. 9B by the fuel cap 1 forced into the filler neck 2, the interfacial separation 58 of the sliding-treated layer 54 from the base 53 or the cracking 59 of the sliding-treated layer 54 is likely to occur around the base portion of the lip 52 which has undergone a high degree of flexural deformation and thereby has a large amount of surface strain.
The interfacial separation of the sliding-treated layer 54 containing a urethane resin as the matrix 56 is apparently due to its insufficient adhesion to the fluororubber base 53 despite the presence of the silane coupling agent 55, and due to its low property of swelling with fuel. Its cracking is apparently due to its insufficient elongation (only about 45% as examined by the inventors of the present invention), and its low property of swelling with fuel.
Japanese Patent Application Publication No. JP-A-57-135871 describes an aqueous fluororubber paint containing a fluororesin (e.g. PTFE) in fluororubber and capable of forming a fluororubber coating having a surface in which the fluororesin is concentrated. International Patent Application Publication No. WO00/56825 describes a fluororubber paint composition containing a fluororesin [a combination of PTFE and a melting fluororesin (e.g. a tetrafluoroethylene-hexafluoropropylene copolymer, or FEP)] in fluororubber and used in a fuel system.